The pasteurisation of milk intended for consumption or as a raw material for cheese making is a well-known process within the dairy industry. Cold milk is heated to a temperature of 63-75° C. and is held at this temperature for a given predetermined period of time. The commonest temperature range, in particular as regards milk for cheese making, is 72-75° C. and with a time duration of 15-20 seconds.
Normally, pasteurisation takes place in a plate heat exchanger and, in order to realise as efficient a heating as possible, one section in the plate heat exchanger is regenerative. This implies that the incoming cold milk is heated up by the already heated milk which in its turn is cooled down by the cold milk.
Despite the pasteurisation process, a number of bacteria normally occurring in the milk, such as Streptococcus thermophilus, survive. These bacteria grow at a temperature of 35-50° C. The milk is at this temperature downstream in a part of the regenerative section of the heat exchanger and bacteria there can grow to a so-called biofilm. After roughly ten hours' production, the biofilm has normally grown so much that it releases from the plate heat exchanger and accompanies the pasteurized milk. Apart from the fact that it is not desirable to have biofilm in consumer milk, milk with biofilm is directly unsuitable to use as a raw material for cheese making. As a result, it is commonest practice to stop production after ten hours and wash the equipment.
Since the intention today is to achieve production times of 20 hours and above in order to render production more efficient, solutions are being sought to remove the biofilm from the downstream part of the regenerative section. One solution is to provide double regenerative sections in the plate heat exchanger and be able to switch between them when one of the regenerative sections is being washed. This is an expensive solution which requires a huge capital investment cost.
In an article by G. C. Knight, R. S. Nicol, T. A. Meekin;“Temperature step changes: a novel approach to control biofilms of Streptococcus thermophilus in a pilot plant-scale cheese-milk pasteurisation plant”; International Journal of Food Microbiology; 93 (2004); pages 305-318, there is a description of how, with recurring temperature increases in the downstream part of the regenerative section, it is possible to prevent biofilm from growing so that it releases from the plate heat exchanger. The temperature increases are realised by raising, in a first heating section, the temperature of the milk which enters into the upstream part of the regenerative section. The described method requires an extra heating section and the supply of more energy to achieve the temporary temperature increases.